ENG-54618: Fix Allocate unique-key conflict and transaction commit issue

[and-bezzir]

Summary

• Fix unique-key conflicts in Allocate: The sequencer's transient INSERT (used to reserve auto-increment IDs) runs on a raw DB connection. When a handler queues DELETEs followed by Allocate on the same table, the old rows are still physically present, causing Error 1062 Duplicate entry. The fix commits pending DML for the target table before the sequencer runs.

• Avoid poisoning the main executor's transaction lifecycle: Uses db.BeginTx directly in a dedicated helper (commitPendingForAllocate) instead of s.Flush/s.Tx, which would trigger txNotifier, mark the main executor's TX as transient, and prevent CommitIfNeeded from committing — silently rolling back all changes.

• Fix Flush not marking statements as executed: Flush now calls MarkAsExecuted() on flushed statements so the main executor skips them via NextNonExecuted.

• Propagate caller context into sequencer: Changed sequencer.New(context.Background(), db) → sequencer.New(ctx, db).

[adranwit]

I'm convince it's the right fix,